The U.S. Environmental Protection Agency (EPA) has estimated that annual emissions of mercury from human activities in the United States were 158 tons during the period of 1994-1995. Approximately 87% of these emissions were from combustion sources. Coal-fired power plants in the U.S. were estimated to emit 48 tons of mercury per year into the air during this period, or about one-third of anthropogenic emissions. The mercury in the flue gas from the power plants has been found in a variety of chemical forms, including elemental mercury and oxidized mercury compounds. A study performed by Electric Power Research Institute (EPRI) indicated that approximately 40% of the mercury emissions from coal-fired power plants were in the oxidized form. The oxidized mercury compounds, such as mercuric chloride, are water soluble and can be removed in a wet scrubber system. However, elemental mercury is not water soluble and easily escapes the wet scrubber.
In addition to the difference in solubility, elemental mercury has higher vapor pressure than the oxidized mercury. Consequently, elemental mercury does not adsorb on sorbents or unburned carbon as readily as oxidized mercury.
Mercury causes environmental and ecological problems. Elemental mercury can be transported over long distance in the atmosphere because of its insolubility, whereas oxidized mercury deposited near the point of emission as a result of its dissolution in fog, cloud, or rain. Once the mercury has deposited on land or water, it can transform into methyl mercury, an organic form, and thereby enter the food chain. Humans are most likely to be exposed to methyl mercury through consumption of fish.
Specific technologies for the control of mercury such as waste incinerators are already proving successful. However, these controls cannot be transposed to coal-fired power plants as the flue gas conditions are different. Flue gas streams from coal-fired power plants are much larger than those from incinerators and the mercury concentrations are much lower, 0.01 ppm in coal-fired plants, at least an order of magnitude lower than incinerator flue gases. Also, the residence time of the flue gases in the air pollution control systems of waste incinerators is longer.
The injection of a sorbent such as activated carbon appears to be one of the most favorable options for mercury control. The sorbent would be injected upstream of existing particulate control devices such as ESP or baghouses. Mercury capture by an injected sorbent depends on the sorbent capacity and mass transfer to the sorbent surface. The capacity and reactivity are affected by temperature and mercury concentration. However, the addition of a sorbent would obviously increase the particulate load in an ESP or baghouse. The total cost of using activated carbon is estimated to add 24$/kW to the plant capital cost and would increase the cost of electricity in the USA by 23%.
Gadkaree et al. U.S. Pat. No. 6,258,334 claimed a method of making and using an activated carbon having sulfur chemically bonded to remove mercury from fluid stream.
Madden et al. U.S. Pat. No. 6,372,187 used an alkaline sorbent injection for mercury control. Alkaline sorbents at low stoichiometric molar ratios of alkaline earth or alkali metal to sulfur of less than 1.0 are injected into a power plant system to remove at least between about 40% and 60% of the mercury content from combustion flue gases.
Felsvang et al. U.S. Pat. No. 5,435,980 patented a method for the removal of mercury from flue gas of coal-fired power plants by adding a chloride or chlorine containing material to the coal before of during the combustion or by injecting gaseous HCl into the flue gas upstream before a spray drying system.
Mendelsohn et al. U.S. Pat. No. 5,900,042 claimed the use of an oxidizing solution to convert elemental mercury to soluble mercury compounds. The aqueous oxidizing solution claimed includes aqueous iodine solution, aqueous bromine solution, aqueous chlorine solution, aqueous chloric acid solution, and combination therefore. This solution is impractical because of the aqueous nature of the oxidizing solution. The oxidant will hydrolyze to form acidic compounds that are not selective for Hg oxidation.
Dangtran et al. U.S. Pat. No. 6,375,909 patented a process of injecting calcium chloride into the combustor and lowering the flue gas temperature in sufficient time to enhance oxidation of mercury and nitrogen oxides into more soluble products prior to their absorption in a wet scrubber.
On Mar. 15, 2005 the EPA promulgated new mercury regulations requiring coal burning power plants to decrease the mercury emissions from the estimated 48 tons a year to 31.3 tons in 2010, 27.9 tons in 2015 and 24.3 tons in 2020. These numbers correspond to a reduction from the current emission levels of 34.8, 41.9 and 49.4 in 2010, 2015 and 2020 respectively.
Cost will be a very important factor in deciding which of the many mercury control options currently under development is chosen for commercialization. Most estimates of the cost of the impending mercury control in the USA assume that activated carbon will be the control method of choice. For a 45% control level, the US DOE has estimated that the annual costs would be between $1.08 and $3.02 billion per year (based on costs for activated carbon). This is based on a cost of 25,000-70,000 $/lb (11,300-31,800 $/kg) of mercury removed.
To solve the aforementioned problems the present invention provides a method using a getter composition that oxidizes volatile and/or insoluble Hg to non-volatile and/or soluble Hg compounds that are then readily removed from gas streams.